In certain lighting applications, the incandescent lamp has been used even though it is much less cost effective than the fluorescent lamp or other gas discharge lamps. A prime example is the exit sign which is commonly used in factories, office buildings, restaurants, apartment buildings, hotels and other such public buildings. Typically, an exit sign has used two incandescent light bulbs of 20 watts each whereas the same sign can be lighted with two fluorescent lamps of 4 watts each. Thus, the fluorescent lamp is much more energy efficient and has an operating lifetime which is greater by a factor of 3 or 4. Accordingly, the operating cost and the maintenance cost, including replacement labor, is much less for a fluorescent lamp in an exit sign than for an incandescent lamp.
The advantages of fluorescent lamps in an exit sign and the like have heretofore been recognized; however, there are difficulties in implementing such a sign which have not heretofore been adequately surmounted. One of the difficulties is that of providing a unit of suitable size and weight which can be easily installed within the limited space of an exit sign fixture to replace the incandescent bulbs. In co-pending application Ser. No. 335,252, filed Dec. 28, 1981 and assigned to the assignee of this invention, a retrofitting unit is described with a base which screws into the existing incandescent lamp socket. A ratchet assembly is provided to allow the unit to be adjusted for retrofitting in most existing exit signs. Thus, either a new or existing exit sign may be provided with fluorescent lamps.
Another difficulty to be surmounted in the use of fluorescent lamps in exit signs is that of controlling the energization of the fluorescent lamps. The conventional fluorescent lamp, namely a low pressure mercury vapor fluorescent electric discharge lamp presents certain problems in the control of energization. Since it is a gas discharge device, a high starting voltage is required to initiate ionization and current limiting must be provided to avoid damage or destruction after ionization has taken place. Such gaseous discharge devices have a negative resistance characteristic and, as the current rises, the resistance decreases thereby allowing more current to be drawn which, without control will eventually destroy the device. Control means must be provided to limit the current to a nondestructive value and yet be sufficiently high to provide the desired intensity of illumination. Generally, a fluorescent lamp is provided with a pair of heated electrodes to promote rapid starting, a transformer to provide high starting voltage, and a ballast to provide for current limiting. Various arrangements of inductors, transformers, capacitors and resistors have been used to provide the ballast for limiting the current flow. The difficulty with a transformer ballast is that it is unduly heavy and bulky for use in an exit sign. The use of a resistive ballast is undesirable because it dissipates a large amount of power and generates undesirable heat.
Electronic ballast circuits have been proposed for use with fluorescent and other gas discharge lamps. The Dendy et al U.S. Pat. No. 3,999,100 discloses a power supply with a switching regulator and a commutator for energizing a metal halide lamp such as that used in television and photographic applications. This circuit avoids the use of an inductive ballast. It uses a switching regulator operated by a frequency of 20 khz and the duty cycle is controlled by a feedback circuit responsive to both DC current and voltage supplied to the commutator. The DC voltage supplied to the commutator is regulated to provide constant power to the lamp. A voltage and current sensing circuit controls the duty cycle of the switching regulator to maintain a constant output power to the lamp.
The Johnson U.S. Pat. No. 3,771,007 discloses a power supply for a special fluorescent lamp which is adapted to provide shaped current waveform in the lamp. The lamp is energized from an AC source through a series choke and an electronic switch such as a triac. The triac is switched by a phase-delayed trigger pulse from a pulse width generating circuit connected with the line volage through a full-wave rectifier.
Other forms of electronic ballast circuits for fluorescent lamps are shown in the Knoll U.S. Pat. No. 4,127,795 and the Kohler U.S. Pat. No. 4,259,614.
A general object of this invention is to provide an electronic energizing circuit for a gas discharge lamp which overcomes certain disadvantages of the prior art and provides an energizing circuit especially adapted for use with fluorescent lamps in exit signs and the like.